Fix x-sort crash when container has a <template> with no .content

[joshhanley]

The Scenario

Placing a flux:chart inside a wire:sort:item (with no wire:sort:handle) breaks drag-to-reorder and throws in the console:

<div wire:sort="handleSort">
    @foreach ($tasks as $task)
        <flux:card wire:key="{{ $task['id'] }}" wire:sort:item="{{ $task['id'] }}">
            <flux:chart :value="[10, 12, 11, 13, 15]">
                <flux:chart.svg>
                    <flux:chart.line />
                </flux:chart.svg>
            </flux:chart>
        </flux:card>
    @endforeach
</div>

The Problem

x-sort auto-detects whether the sort container uses drag handles. If no handle element is found directly, it scans nested <template> elements for one:

useHandles: !! el.querySelector(handleSelector)
    || Array.from(el.querySelectorAll('template')).some(tmpl => tmpl.content.querySelector(handleSelector)),

querySelectorAll('template') matches every element with tag name template regardless of namespace, but .content only exists on HTMLTemplateElement. Flux's chart hydrates its SVG from an HTML <template> and the hydrated <svg> contains nested <template> elements that were parsed in the SVG namespace, so they're SVGElement instances with no .content property. The unconditional .querySelector call on tmpl.content throws.

The Solution

Guard the property access with optional chaining so non-HTMLTemplateElement matches are skipped:

|| Array.from(el.querySelectorAll('template')).some(tmpl => tmpl.content?.querySelector(handleSelector)),

Fixes livewire/flux#2578